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Selective synthes

aDepartment of Chemistry, Hakim Sabzevari

E-mail: b.maleki@hsu.ac.ir; malekibehrooz@

+98-571-4002643bYoung Researchers and Elite Club, Kerma

Kermanshah, IrancInstitute of Industrial Chemistry, Iranian

Technology, Tehran, IrandDepartment of Chemistry, Payame Noor Un

Cite this: RSC Adv., 2014, 4, 40505

Received 30th June 2014Accepted 18th August 2014

DOI: 10.1039/c4ra06132b

www.rsc.org/advances

This journal is © The Royal Society of C

is of sulfoxides and sulfones fromsulfides using silica bromide as the heterogeneouspromoter and hydrogen peroxide as the terminaloxidant

Behrooz Maleki,*a Saba Hemmati,b Alireza Sedrpoushan,c Samaneh Sedigh Ashrafia

and Hojat Veisid

Silica bromide as a heterogeneous promoter and reagent is prepared from the reaction of silica gel with

PBr3 as a non-hydroscopic, filterable, cheap, and stable yellowish powder that can be stored for months.

The results show that silica bromide is a suitable and efficient promoter for the chemoselective oxidation

of sulfides to the corresponding sulfoxides or sulfones in the presence of 30% H2O2 in acetonitrile. The

excellent yields, heterogeneous conditions, simplicity, compatibility with a variety of functionalities, and

ease of isolation of the products make our procedure a practical alternative.

Introduction

In order to perform chemical reactions in heterogeneous mediarather than homogeneous, a number of ideas was deliberatelysuggested by R. B. Merrield1 for use in polypeptide synthesisand by R. L. Letsinger2 for polynucleotide synthesis. It wasdemonstrated that the classical idea that chemical reactionsshould be performed in a completely homogeneous mediumwas not necessarily correct and that reactions can be accom-plished even if one of the substrates was insoluble in the reac-tion media. Uncatalyzed reactions of neat reactants are ofelemental importance in chemistry and one of the mostpreferred transformations. The methodology of some organicsynthesis has been revolutionized by this idea in that thenormal procedures associated with the workup of a chemicalreaction are obviated and replaced by a simple ltration step.1,2

This general advantage of solid-phase synthesis has beenparticularly exploited in polypeptide synthesis3 where a poly-peptide is synthesized in a repetitive sequential manner on thesolid phase and the nal products are only liberated from thepolymer in a nal cleavage reaction. Another approach to usingpolymer supports in organic synthesis was outlined by Fridkin,Patchornik, and Katchalski,4 who showed that a polymer-boundreagent could be used in heterogeneous reaction in such a way

University, Sabzevar 96179-76487, Iran.

gmail.com; Fax: +98-571-4410300; Tel:

nshah Branch, Islamic Azad University,

Research Organization for Science and

iversity(PNU), 19395-4697 Tehran, Iran

hemistry 2014

that excess reagents or by-products remain attached to theinsoluble resin. These unwanted materials are then removed byltration and the pure product is isolated from the ltration.5–11

Many specic advantages of using insoluble resins assupports, reagents, or catalysts have been reviewed,12,13 theseinclude the simulation of high-dilution14,15 or pseudodilution16

conditions, the shhook and concentration principle,17 selec-tive intrapolymeric reactions,18 bulk and steric effects of thepolymer backbone in asymmetric synthesis,19 the stabilizationof reactive substances,20,21 and the elimination of volatilemalodorous reagents.22,23

Oxidation of suldes is the most straightforward method forthe synthesis of sulfoxides and sulfones. Sulfones are valuablesynthetic intermediates for the construction of various chemi-cally and biologically signicant molecules in particular.24,25

Sulfones represent an important course group of compoundsdue to their properties and reactivity. The direct oxidation ofsuldes is an important and widely studied reaction for thepreparation of sulfoxides and sulfones. However, the oxidationof suldes to sulfones has been much less investigated ascompared to the oxidation of suldes to sulfoxides. For theselective oxidation of suldes to sulfoxides and sulfones, a rangeof oxidants have been studied. In addition, H2O2 was frequentlyutilized in combination with different catalysts such as MoO3,26

CH3ReO3,27 dioxo-molybdenum(VI) complex,28 NH4Cl,29 poly-oxometalate-cored dendrimers,30 silica–vanadia catalyst,31 silicasulfuric acid,32 heterogeneous TiO2,33 tetra-(tetraalkylammoni-um)octamolybdate,34 immobilized molybdenum hetero-polyacid,35 TaC and NbC,36 sodium tungstate/PTC/phenylphosphonic acid,37 MoO2Cl2,38 Cp0Mo(CO)3Cl,39 cer-ium(IV) triate,40 cyanuric chloride,41 [SeO4{WO(O2)}2]2,42

composite oxide catalyst LiNbMoO6,43 carbon-based solid acid,44

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1,3,4-triazo-2,4,6-triphosphorine-2,2,4,4,6,6-tetrachloride,45H3BO3,46

H5IO6,47–49 ozone,50 an aqueous NaOCl,51 HOF$CH3CN,52 oxygen/2-methylpropanal,53 NaIO4,54 oxone,55 ZnCl2/DBU,56 [(C18H37)2-(CH3)2N]3[SiO4H(WO5)3],57 TsOH/PhI(OAc)2,58 PyHBr3/TBN,59

(C19H42N)2[MoO(O2)2(C2O4)]$H2O,60 poly(N-vinylpyrrolidone)and poly(4-vinylpyridine),61 silica-based tungstate interphase,62

and 30% H2O2.63

Besides extended reaction times, some of these processessuffer from drawbacks, such as elevated temperatures, unde-sired side reactions occurring on other functional groups, theuse of hazardous peracids, and toxic metallic compounds thatgenerate waste streams. However, a promoter was still requiredin those processes. Therefore, simple, convenient and envi-ronmentally benign methods for the oxidation of suldes tosulfoxides or to sulfones are still highly desired.

Result and discussions

In continuation of our research on new synthetic methods inorganic synthesis,64–73 herein, we report the application of silicabromide as a selective and efficient heterogeneous promoter forthe oxidation of suldes into sulfoxides and sulfones usingH2O2 in acetonitrile at room temperature (Scheme 1).

Heterogeneous reagents have gained signicant attractiondue to economic and environmental considerations. They canbe handled conveniently and removed from the reactionmixture, thus making experimental procedures simple.74–77

Silica bromide as heterogeneous promoter and reagent isprepared from reaction of silica gel with PBr3 as a non-hydro-scopic, lterable, cheap, and stable yellowish powder that canbe stored for months.78

A preliminary study with benzyl phenyl sulde (2 mmol), 30%H2O2 (2 mmol), and silica bromide (2 mmol Br per g silica) as amodel reaction quickly established that this oxidation protocolproduced the anticipated sulfoxide in excellent yield (98%) and ina short reaction time (5 min) in CH3CN at room temperature.Then, the inuence of different reaction parameters (amount ofreagents, temperature and solvent) on the efficiency of the oxida-tion process was evaluated in this model reaction. The oxidationwas investigated in various solvents such as toluene, THF, CH3CN,CH3NO2, CH3OH, C2H5OH, CH3COOH, CH2Cl2 and CCl4 using30%H2O2 (2mmol), and silica bromide (2mmol Br per g silica) at

Scheme 1 Conversion of sulfides into sulfoxides and sulfones.

40506 | RSC Adv., 2014, 4, 40505–40510

room temperature. Aer screening different solvents, it was foundthat CH3CN is proved to be the optimum solvent and was used insubsequent optimization studies. At lower concentration ofhydrogen peroxide (1 mmol), the reaction took a long time (10min) and lowyield (86%). The increasing of thehydrogenperoxide(3 mmol), loading did not signicantly affect the yield and time.The reaction was also monitored with different silica bromideloading, at a lower silica bromide loading (1 mmol) the reactiontook longer time (8min) and lower yield (91%). Further increase inamount of silica bromide (3mmol) in thementioned reaction didnot has signicant effect on the product time and yield. It was alsofound that withoutH2O2, and silica bromide alone cannot oxidizesuldes to sulfoxides. Therefore, A ratio of (sulde–H2O2–SB;2 : 2 : 2) was found to be optimum for the complete conversion ofsuldes to sulfoxides in CH3CN at room temperature.

In order to generalize the scope of the reaction, a series ofstructurally diverse suldes were subjected to oxidation underthe optimized reaction conditions, and the results are pre-sented in Table 1. Oxidation of allyl phenyl, alkyl aryl, arylbenzyl, dialkyl, diaryl, cyclic, and heterocyclic suldes producedexcellent yields of the corresponding sulfoxides. It is notablethat the suldes were chemoselectively oxidized in the presenceof oxidation-prone functional groups, such as OH and C]C.

Encouraged by these results, we studied the completeconversion of suldes (via sulfoxides) into sulfones. Initially, weoptimized amount of silica bromide and hydrogen peroxide foroxidation of benzyl phenyl sulde (2 mmol) to correspondingsulfone. The best result was obtained with a ratio of (benzylphenyl sulde–H2O2–SB; 2 : 2 : 2) in CH3OH at 40 �C. Toinvestigate the versatility of the system, the reaction of oxida-tion suldes (via sulfoxides) into sulfones was carried out inCH3OH at 40 �C. All reactions proceeded efficiently within25–100 minutes at 40 �C to provide the corresponding sulfonesderivatives in excellent yields ranging from 84–95% (Table 1).

A possible mechanism for oxidation of the sulde into thecorresponding sulfoxides and sulfones using H2O2 in the pres-ence of silica bromide is shown in Scheme 2. Nucleophilic attackof H2O2 on silica bromide leads to intermediate 1 in which theoxygen atom is more electrophilic. Next, nucleophilic attack ofsulde 2 on this intermediate gives intermediate 3 followed bythe abstraction of hydrogen to yield the corresponding sulfoxide4. Sulfoxide 4 reacts with the intermediate 1 to form 5, whichfollows the abstraction of hydrogen produces sulfone 6.

Experimental

Chemicals were obtained from Merck and Fluka chemicalcompanies. Nuclear magnetic resonance spectra were recordedon a Brucker Advanced DPX-250 MHz spectrometer using tetra-methylsilane as internal standard. Infrared spectra were recor-ded onaPerkin-Elmer 781 spectrometer. The plate silica gel usedfor the preparation of silica bromide was type 60 (15–40 mm).

Procedure for preparation of silica bromide

To silica gel (40 g) and dry toluene (80 mL) in a round bottomedask equipped with a condenser and a drying tube, was added

This journal is © The Royal Society of Chemistry 2014

Table 1 Conversion of sulfides to sulfoxides or sulfones

Entry Substrate

Timea (min) Yieldb (%) Timea (min) Yieldb (%)

Sulfoxide Sulfone

1 10 91 75 91

2 10 94 90 92

3 10 92 80 90

4 3 99 30 95

5 5 98 25 94

6 7 96 30 92

7 5 92 40 90

8 7 94 35 94

9 7 89 45 84

10 5 91 40 90

11 7 94 95 89

12 7 89 50 85

13 10 92 40 90

14 10 94 60 82

15 5 95 70 92

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Table 1 (Contd. )

Entry Substrate

Timea (min) Yieldb (%) Timea (min) Yieldb (%)

Sulfoxide Sulfone

16 10 96 100 90

a The course of reaction was checked by TLC or GC. b Yield of isolated pure sulde.

Scheme 2 Proposed mechanism.

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phosphorus tribromide (75 g, 0.21 mol) and reuxed for 18 h.Aer cooling, the product was ltered and washed, rst, withdry 1,4-dioxane (2 � 20 mL) and then with dichloromethane(2 � 20 mL). The yellowish product was kept in dessicator. Theamount of bromosilyl group (2.2 mmol of Br per g silica) isdetermined by a standard method.78

General procedure for the preparation of sulfoxides

To a mixture of sulde (2 mmol) and yellowish silica bromide(2 mmol) in acetonitrile (5 mL) was added 30% H2O2 (2 mmol).The mixture was stirred at room temperature for the appro-priate period of time until complete consumption of the start-ing material as observed by TLC. Aer completion of thereaction, the reaction mixture was ltered to separate the whiteSiO2. Then, the ltrate was extracted with EtOAc (10 mL) andwashed with Na2CO3 (20%, 10 mL). The organic layer was driedover anhydrous Na2SO4. The solvent was removed underreduced pressure to afford the desired product without furtherpurication (Table 1).

General procedure for the preparation of sulfones

To a mixture of sulde (2 mmol) and yellowish silica bromide(2 mmol) in methanol (5 mL) was added 30% H2O2 (2 mmol) at40 �C. The mixture was stirred at 40 �C for the appropriateperiod of time until complete consumption of the startingmaterial as observed by TLC. Aer completion of the reaction,the reaction mixture was ltered to separate the white solidSiO2. Then, the ltrate was extracted with EtOAc (10 mL) and

40508 | RSC Adv., 2014, 4, 40505–40510

washed with Na2CO3 (20%, 10 mL). The organic layer was driedover anhydrous Na2SO4. The solvent was removed underreduced pressure to afford the desired product without furtherpurication (Table 1).

Conclusion

In conclusion, we have shown that silica bromide (SB) asheterogeneous reagent can promote the selective oxidation of avariety of suldes into the corresponding sulfoxides andsulfones in the presence of H2O2 as the terminal oxidant. Thisreagent is prepared from reaction of silica gel with PBr3 as anon-hydroscopic, lterable, cheap, and stable yellowish powderthat can be stored for months. Although the literature reports anumber of procedures for the oxidation of suldes, the excellentyields, heterogeneous conditions, simplicity, good availabilityof starting materials, compatibility with a variety of function-alities, and ease of isolation of the products make our procedurea practical alternative.

Acknowledgements

We are thankful to Payame Noor University (PNU), and HakimSabzevari university for partial support of this work.

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